Reliable and accurate bolt tension gages are an essential tool in the Kennedy Space Center (KSC) operational environment to determine the amount of preload in critical bolts and studs that are located in both ground support equipment and flight hardware. Experience at KSC has shown that commercially available ultrasonic bolt gages have problems taking required measurements. Existing bolt gages perform adequately in most instances, but can produce unacceptable errors and uncertainties when performing tension measurements in some configurations of flight hardware. These errors and uncertainties often result from interference due to multiple return waveforms from an ultrasonic pulse launched through the bolt or stud being measured. These errors are caused by apparent changes in amplitude, peak splits, peak merges, and distortion in return waveforms being analyzed and the bolt gage's inability to compensate for these phenomena. Other factors that affect the accuracy of a measurement include the tensioning process which may cause changes in geometry and material characteristics, temperature changes between unloaded and loaded conditions, contamination between the transducer and the surface of the item under test, contact pressure between the transducer and the item under test, and changing or removing and replacing a transducer during the process.
Most commercial bolt gages operate by determining the time-of-flight (TOF) of an ultrasonic return pulse in a bolt or stud. The TOF is measured by selecting a single feature of the item under test. Typically, an ultrasonic transducer 110 is mounted at one end of the item under test, such as a bolt 100, a pulse is launched through the item by a pulser circuit 112 simultaneously, the electronic clock is started. The time is measured for a return signal, or echo pulse as detected by a receiver 113 (See FIG. 1A). The traditional gage uses a single waveform feature such as the slope and trigger level setting as the requirement for stopping the clock. This single feature TOF is the basis for comparison and measurement. The difference between an unloaded (zero applied tension) and loaded TOF measurement for a particular bolt or stud is compared in terms of nanoseconds. Since each tension state has a unique sound speed associated with it, the nanoseconds interval can be converted to an ultrasonic length. Provided there is no permanent deformation in the item under test, the preload of the bolt or stud may be calculated either by using Hooke's Law or using a pre-determined plot of ultrasonic length (or TOF) versus load, taking into account the speed of sound change in the material under load and compensating for the other factors, such as temperature.
That is, preload can be determined by correlating the first return waveform of a bolt in both an unloaded and a loaded state. The amount of the time shift measured between these states is proportional to the preload tension in a joint coupled by the bolt. The two time measurements are compared to determine how far the waveform has been displaced in time. The time difference is proportional to the stretch in the bolt. This method and traditional bolt gages rely on an arbitrary zero or starting point. The resulting TOF includes the length for an equipment cable, transducer and couplant delays, as well as any delays in the measuring device. These are all system related external errors and can be subtracted out. Thus, an unloaded length or TOF is obtained and recorded.
In a second method, the first and second return (echo) waveforms are correlated to produce the actual TOF in the item under test rather than including the elements of the total measurement system. The first return waveform is used as the reference for the second return waveform. By considering the time between the first and second returns, the external variables in the first TOF method and typical bolt gages are eliminated. A reference table for each bolt, which plots tension versus delay time, is consulted to determine bolt tension. This plot is typically slightly parabolic in shape.
A problem with bolts in critical applications is that they are usually very lightweight in comparison to the load they bear. This creates a maximum preload tension value. That is, if the bolts are over-tensioned before load, they can yield or break upon application of the load. Likewise, there is also a minimum preload to prevent a joint from separating. It is well known that simple torque preload techniques result in a +/30% variation in tension preload, and that this is usually unacceptable in critical joints.
In the 30 years since the ultrasonic bolt gage was first invented, many methods have been tried to increase the level of repeatability and confidence in the accuracy of readings taken with these instruments. When a modern ulrasonic bolt gage and its operator are performing properly, these instruments can measure to an accuracy of +/- 2%. Unfortunately, on the all too often occasions when a gage reading "jumps peak," a user's confidence in the data collected is significantly reduced. A significant amount of peak jump error is a result of the reliance on one single feature to meet the same measurement and trigger criteria in the waveform in both unloaded to loaded conditions. Thus, current bolt gages while accurate are not reliable. For example, when a current bolt gage is used in the critical situation such as testing bolts on a space station or rocket, a reliability error of 10 to 30 percent is unacceptable. This unreliability is more than an annoyance, it can create an unacceptably dangerous situation. If a technician has 10 critical bolts which must be tensioned within a particular window, and the bolt gage used as a plus or minus 30% error one time in 10, the technician cannot insure that all 10 bolts are correctly tensioned.
See FIG. 1B where the first ultrasonic wave, A, of a slack bolt has a first zero-crossing 114 after the signal crosses the threshold level. Waveform B is an echo pulse of the bolt under tension. Because the waveform is deformed, in this illustration, simply attenuated, the zero-crossing 116 following the threshold level does not correspond to that of the first waveform. This creates substantial errors when calculating tension depending upon the single waveform feature. This is a common effect of tensioning a bolt.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a more reliable ultrasonic bolt gage.